1. Field of the Invention
The present invention relates to liquid crystal display devices, and particularly relates to a liquid crystal display device employing a liquid crystal modulation element.
2. Description of the Related Art
In general, liquid crystal projectors and liquid crystal displays are known as examples of a liquid crystal display device employing a liquid crystal element. Such liquid crystal elements employed in liquid crystal display devices include TN (twisted nematic) liquid crystal elements serving as transmissive liquid crystal elements or VAN (vertical alignment nematic) liquid crystal elements serving as reflection liquid crystal elements.
Such a liquid crystal element is configured such that liquid crystal fills a portion between a first transparent substrate having a transparent electrode (common electrode) and a second transparent substrate having transparent electrodes (pixel electrodes), lines, and switching elements, for example, which constitute pixels. The portion including the liquid crystal is particularly referred to as a liquid crystal layer.
The liquid crystal element is used to form an image by controlling a polarization state of light which is transmitted through the liquid crystal. This is carried out by making use of a characteristic in which an electric field is generated in the liquid crystal layer by controlling voltages between electrodes of the liquid crystal element so that an alignment direction of liquid crystal molecules is changed and the polarization state of the light transmitted through the liquid crystal is changed.
However, charged particles are included in the liquid crystal layer and an outer-wall member surrounding the liquid crystal layer, for example. When the liquid crystal is driven in a high-temperature environment, in particular, the charged particles drift (move). The charged particles serving as direct-current electric-field components in the liquid crystal layer are attached to an alignment layer or an electrode interface of a liquid crystal layer interface, and the charged particles drift and are deposited along the alignment direction of the liquid crystal molecules.
Furthermore, in liquid crystal elements having organic alignment layers, when the liquid crystal is driven in a high-temperature environment, in addition to drift of charged particles, charged particles are newly generated since organic members such as an alignment layer, liquid crystal, and a seal member are broken down due to light which is incident onto the liquid crystal elements. These charged particles also serve as direct-current electric-field components in a liquid crystal layer, are attached to an alignment layer or an electrode interface of a liquid crystal layer interface, and the charged particles drift and are deposited along the alignment direction of the liquid crystal molecules.
If an effective electric field to be applied to the liquid crystal is changed due to the charged particles deposited in a certain region of the liquid crystal layer, the polarization state cannot be controlled as desired and quality of the image is degraded.
Measures to address such a problem have been proposed.
For example, a method for separating ion which causes a sticking phenomenon from an alignment layer or an electrode interface by making at least one of a potential of a pixel electrode and a potential of a counter electrode of a liquid crystal cell be a ground level while an image display operation is not performed has been proposed (refer to Japanese Patent Laid-Open No. 2005-55562, for example). Furthermore, a method for arranging a region of ion-trap electrodes in a non-display region of a liquid crystal element, and applying a direct current voltage to the ion-trap electrodes, so that impurity ion in the region of the ion-trap electrodes included in the non-display region which is not used for image display is absorbed has been proposed (refer to Japanese Patent Laid-Open No. 8-201830, for example).
However, when the method disclosed in Japanese Patent Laid-Open No. 2005-55562 is used, switching portions used to bring the counter electrode to the ground level should be included in circuits of the liquid crystal element. Therefore, the number of steps of manufacturing of the liquid crystal elements is increased. Furthermore, when the counter electrode is merely brought to the ground level, a force for separating the ion being attached to the alignment layer and the electrode interface is smaller than the coulomb force, and therefore, only small effect is attained.
Furthermore, when the method disclosed in Japanese Patent Laid-Open No. 8-201830 is employed, since the ion-trap electrodes which suck the ion is newly arranged on the non-display region, the number of steps of manufacturing the liquid crystal element is increased. In addition, since impurity of the ion is absorbed by the coulomb force and the coulomb force is in inversed proportion to the squared of a distance, the ion generated in portions separated from the ion-trap electrodes cannot be effectively absorbed.